The present invention relates to precious metal alloys and, more particularly, to such alloys which are used for making dental restorations.
Alloys utilized for making dental restorations are required to have a high degree of biocompatability or inertness when exposed to the conditions in the mouth, and good physical properties so that they will provide a long lived restoration. In addition, they should not adversely affect the color of the porcelain coating which is fired thereon so that the restoration will blend with the appearance of the natural teeth.
For many years, gold and platinum alloys had been utilized for such restorations, but the escalating costs of those metals produced a trend toward use of non-noble metal alloys and to use in precious metal alloys of palladium which was considerably lower in cost. Initially, palladium/silver alloys were favored because of their lower cost and simulation of the appearance of platinum alloys. However, some palladium/silver alloys exhibited a tendency to discolor the porcelain coatings fired thereon, i.e., the so called "greening" effect. Illustrative of palladium/silver alloys for dental restorations are those described in Schaffer U.S. Pat. No. 4,350,526 and Ingersoll U.S. Pat. Nos. 3,929,474 and 3,929,475.
Schaffer U.S. Pat. No. 4,387,072 addressed the greening problem by a silver-free alloy which essentially included palladium, gallium and copper or cobalt. Alloys of this type were highly successful in replacing palladium/silver alloys because they did not demonstrate the greening effect. Unfortunately, this alloy has a tendency to produce a dark oxide during firing of the porcelain and this dark oxide may produce an undesirable effect in trying to match the restoration and the natural teeth.
Recently, Groll et al U.S. Pat. No. 5,298,218 has attempted to address the dark oxide problem by incorporating with in a palladium/gold alloy and 9-14 percent by weight of modifying elements comprising gallium, tin, and indium with a number of other metals as optional additives. Although the patentees indicate a substantial improvement in the darkness of the oxide based upon apparent visual comparison, the oxide formed on the surface of the casting during porcelain firing still is relatively dark and has a tendency to discolor some translucent porcelains.
Accordingly, it is an object of the present invention to provide a novel precious metal alloy for dental restorations which will form a light colored oxide during firing.
It is also an object to provide such an alloy which exhibits a good balance of properties and which may be easily cast.
Another object is to provide a dental restoration utilizing a casting of the novel alloy and no readily discernible discoloration of the porcelain by the oxide film formed on the casting.